1. Field of the Invention
This invention relates to control systems for motors and, more particularly, to a motor control system for a unidirectional motor having improved motor dynamic braking.
2. Description of the Prior Art
Dynamic braking for electric motors has been in widespread use for over eighty years. The 1915 Edition of the Standard Handbook for Electrical Engineers, McGraw-Hill Sec. 15,-464, pages 1304-05 defines dynamic braking as follows: ". . . the retardation of a machine by a motor acting as a generator. The subject of dynamic braking may be divided into two parts as follows: first, for the purpose of effecting a quick stop; second, for the retardation of a descending load ."
Recently, an article in Machine Design, June, 1992 at page 484 notes that: "Dynamic braking of dc motors occurs when the armature terminals are disconnected from the power supply and then connected together, while the field is energized. The resultant armature current from the counter-emt voltage creates a magnetic field that opposes motor rotation."
In a field as old and commercial as dynamic braking of electric motors, the patent and technical literature is replete with examples of improvements. For example, U.S. Pat. No. 5,184,049 dated Feb. 2, 1993 to Kiuchi utilizes a switching mechanism for short-circuiting the power terminals of the stator windings of a three-phase motor once a brake signal is issued from the brake circuit. This rapidly brings the motor to a full stop. U.S. Pat. No. 3,260,908 dated Jul. 12, 1966 to Bostwick arrests the rotation of an AC motor by reversing the motor armature connections through an elaborate circuit to effect braking. U.S. Pat. No. 3,463,991 dated Aug. 26, 1969 to Yuminaka et. al. utilizes a rectifier bridge and a single SCR switch to effect braking of a motor. U.S. Pat. No. 2,575,021 dated Nov. 13, 1951 to Leitch et. al. describes a fairly complicated system in which motor field connections are reversed to obtain braking.
All of the commercial systems for dynamic braking tried to date have drawbacks, some of which are a lock of consistent operation and reliability and problems encountered with residual fields and delays in motor response and thus braking response. For some applications very reliable and consistent operation is a requirement and a typical example for such an application is a remote switch for a high-voltage circuit.